1. The Field of the Invention
The present invention relates to a biological observation apparatus and a method and, more particularly, to a biological observation apparatus and a method which use a sound wave and light to obtain characteristic information indicative of internal states of an object to be examined.
2. Related Art
In recent years, as optical tomographic imaging for a living body, there have been known optical CT (computed tomography), optical coherence tomography (hereinafter, referred to as “OCT”), and photoacoustic tomography.
The optical CT utilizes near-infrared light of a wavelength ranging from 700 nm to 1200 nm, which is comparatively weakly influenced by scattering in a living body. Therefore, the optical CT enables obtaining tomograms of deep parts in the living body, such as up to several centimeters under a mucous membrane.
OCT, which utilizes interference, can obtain tomographic images of a living body up to a depth of about 2 mm with high resolution (several μm to several tens of μm) in a short time. The OCT has already been put into practice in diagnosing retinopathy in the ophthalmic field. Therefore, the OCT has attracted very keen interest in the medical world.
Although the optical CT can obtain information on a deep part of a living body, its spatial resolution is as low as several millimeters. In contrast, it is difficult for the OCT to perform observation at a depth of about 2 mm or more under a mucous membrane and to provide a good quality image of tumor tissue, such as a cancer. This is because the optical coherence is greatly disturbed by the influence of absorption of light by blood or strong scattering in the deep parts of a living body and tumor tissue.
Considering this situation, a technique for obtaining internal information from a living body other than using optical CT and OCT has been disclosed in Japanese Patent Laid-open Publication No. 2000-88743. In this technique, ultrasound waves and light having a single wavelength are radiated into a target portion inside a living body in order to detect how much the light is scattered by the ultrasound wave in the target portion. Thereby, information on the target portion of the living body can be obtained.
An optical measurement apparatus having the above-described configuration disclosed in Japanese Patent Laid-open Publication No. 2000-88743 obtains a value of how much the light is scattered. The value depends on indexes of refraction of a nucleus, cytoplasm and the like, which are substances existing in a living body. The publication states that each index of refraction of the substances constituting the living body is approximately 1.4.
The optical measurement apparatus uses light having a single wavelength to detect how much light is scattered. In consequence, due to the above-mentioned factors, the apparatus can obtain the information depending on how much the light is scattered only in a narrow dynamic range, thereby generating a tomographic image in which the difference of brightness between a bright section and a dark section is small.
As a result, for example, the optical measurement apparatus outputs a tomographic image in which the shape of tumor tissue is difficult to identify. Consequently, there is a problem that the operator is burdened during an observation of a target portion using the tomographic image.